Waterborne paint testing and standards:hardness
Hardness is one of the most important physical properties of a coating. Coating hardness reflects the coating's ability to resist constant dents, scratches, scribes, and penetrations caused by a hard object. Testing the hardness of materials with different methods will give different results because the testing methods are for different amounts of materials, and each method has its own scale to determine the hardness characteristics of the material. Due to different emphasis, there is no absolute scale that can unify the hardness measured by different methods, but the results obtained by one method are comparable.
The commonly used hardness test method for wood coatings is the pencil hardness method. The hardness side measured by pencil hardness test focuses on the coating's resistance to dents, scratches, and scratches. Although some people think that the technical content of pencil hardness is very low, but because it is simple and practical and can be directly measured on wood, the pencil hardness method is still widely used in the coating industry. Before the preparation of the sample, the wood should be polished, smoothed and cleaned. The dry thickness of the coating should be between 25 and 40 μm. It should be dried for 7 days after painting. The film thickness and long drying time affect the hardness test results.
The set of pencils used for the test consisted of the hardest 9H to the softest 6B. Commonly used are 4H, 3H, 2H, H, F, HB, B, 2B, 3B, 4B, 5B, and 6B. F is the intermediate hardness, etc., "H," means "Hardness", "B" stands for "Blackness" The hardness of a pencil is determined by the ratio of graphite and clay used in the manufacture of lead cores. The darker, the softer; the more clay, the harder the lead core made. The hardness number of the pencil is used to indicate the hardness of the paint film.
Test the pencil first to remove the wooden rod to expose a lead of about 3mm in length. The pencil should be ground vertically on the sandpaper to make a flat surface, and the edges should be sharp. Hold the pencil so that it is 45 degrees from the paint surface to be tested. Corner, apply pressure, and continuously draw the pencil lead on the paint surface. Each hardness pencil is measured 5 times on the sample, each time with a new sharp-lead lead, find the pencil number that scratched or scratched the paint at least 2 out of 5 times, and then use a soft The number one pencil test, if you get no more than one sting or scratch, the latter number is set as the hardness of the coating. In order to eliminate the deviation caused by uneven force and angle of holding the pencil, various pencil hardness testers have been put on the market.
The range of pencil hardness grades has a wide range, which cannot reflect the small hardness difference between paint lacquers, that is, the actual soft hardness of paint films with the same pencil hardness may be significantly different. However, the pencil hardness comprehensively considers the coating's resistance to dents and scratches, which has great practicality.
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